What is the consequences of overflow of transmission fluid?

I would like to buy a 2001 Chevy Impala (3.8L engine V6) with 93600 miles. It was found that there were overflow of transmission fluid after mechanic check. The color of transmission fluid seems deep brown. It is estimated that this overflow lasted about almost one year. But the owner did not know that.

This car drives very smoothly (gear change, accerleration, et al).

So I am wondering what the potential damage to the car (transmission system or other parts) in such situation? It is good or not to buy this vehicle?

Thanks. I appreciate all kind of information and suggestions.